Navigating and controlling a spacecraft for rendezvous with another spacecraft in a circular orbit is a difficult and complicated process. Further compounding this process are the precise nature and errors of the measurements for navigating and controlling a spacecraft for rendezvous with a spacecraft in a circular orbit.
Spacecraft navigation systems typically include multiple components. These systems may include various types of sensors to provide measurements for the component systems, such as visible and infrared sensors, laser range finders, mechanical or ring laser gyros, accelerometers, Sun sensors, star trackers, and GPS (global positioning system) receivers. These instruments often are misaligned and their measurements are corrupted with noise. Kalman filters may be used to provide estimates of relative position, velocity, attitude and rates.
When two spacecraft in near-Earth circular orbits are within a distance of approximately 10 km, guidance policies for a chaser satellite may be based on linear Clohessy-Wiltshire equations. The calculation of guidance delta-velocities utilized by the Clohessy-Wiltshire equiations for rendezvous and proximity operations requires position and velocity estimates of the chaser satellite relative to the target satellite.
For this process to be automated, a laser range finder (LRF) instrument, mounted on a chaser satellite, may be pointed to a target satellite so that the range between the two spacecraft may be measured. However, because of the narrow width of the laser beam and small target size, the laser range finger (LRF) must be pointed accurately at the target to approximately 90 μrad/axis, or 1σ, in both azimuth and elevation from a distance of approximately 10 km. This stringent pointing accuracy must be accomplished with a collimated visible sensor while multiple factors further complicate the process, such as: the relative position and velocity of the chaser satellite are not accurately known, the gyroscope and star tracker measurements of the chaser satellite attitude are noisy, the line-of-sight (LOS) angle measurements from the focal plane of the visible sensor are noisy, sensors may be mutually misaligned, and a reaction wheel attitude controller may be turning the chaser satellite about its roll axis, parallel to the visible sensor boresight and the laser range finder (LRF) beam, so that two solar arrays of the spacecraft, each with only one articulation degree-of-freedom about the pitch axis, maintain Sun-pointing. Furthermore, due to the occasional poor lighting of the target satellite, the visible sensor measurements can be interrupted. If this occurs, the laser range finger (LRF) pointing will be lost. But, when the visible sensor measurements resume, the required pointing accuracy must be quickly regained.
An improved technology is needed to address the above problems for relative navigation, attitude determination, pointing and tracking for spacecraft rendezvous in a circular orbit.